At universities where introductory physics labs are taught by graduate student teaching assistants (TAs), there is a need for specialized professional development for those TAs. This paper presents a specific instantiation of a model for lab TA professional development that uses a combination of cognitive apprenticeship and expectancy-value theories as its framework. We describe how our model was implemented in the lab TA professional development program, which included reflections, role playing, and other pedagogical activities offered through weekly meetings. Our evaluation included an analysis of TA writing and interactions with students alongside informal observations and interviews. We discuss the importance of accounting for TAs' interest and self-efficacy development in teaching the labs, as well as the challenge of motivating TAs who have very low initial levels of interest in supporting student learning. We find that many TAs in our lab TA professional development program demonstrated an improvement in TA performance in supporting student learning. Given that the professional development activities require only a modest investment of time, these positive results suggest that the model of lab TA professional development may be usefully adopted and adapted at other institutions where introductory labs are led by graduate student TAs.